1. Field of the Invention
The present invention relates in general to a method for isotope separation of thallium, and more particularly to laser isotope separation of thallium using a laser beam.
2. Description of the Prior Art
In nature, thallium contains two isotopes, including 203Tl (29.5%) and 205Tl (70.5%). 203Tl is used as a source material to produce 201Tl radioisotope. 201Tl is produced in a cyclotron by irradiating the enriched 203Tl target with a 19-31 MeV proton beam according to the 203Tl(p, 3n) 201Tl reaction. 201TlCl is a radiopharmaceutical for SPECT (single photon emission computerized tomography) to diagnose heart diseases and tumors.
The unique commercialized method used to separate thallium isotopes is referred to as the electromagnetic method. In principle, the technique is simple. When passing between the poles of a magnet, a monoenergetic beam of ions of naturally occurring thallium splits into two streams according to their momentum, one per isotope, each characterized by a particular radius of curvature. Collecting cups at the ends of the semicircular trajectories catch the homogenous streams. It is generally agreed that electromagnetic separation represents the most versatile technology with the greatest experience base of any separation technology, but unfortunately it has low throughput and high operating cost.
Laser isotope separation (LIS) technique employing isotope-selective photoionization process of atoms has been said to be an alternative to overcome the demerits of electromagnetic method. The technique has been demonstrated on a number of elements, including mercury, gadolinium, and other elements. U.S. Pat. No. 4,793,307 (1988), U.S. Pat. No. 5,202,005 (1993), U.S. Pat. No. 5,376,246 (1994), and U.S. Pat. No. 5,443,702 (1995) are related to laser isotope separation of mercury(Hg), gadolinium(Gd), zirconium (Zr), and erbium(Er) atoms, respectively. Japanese patent publication No. 1999-99320A suggests a new photoionization pathway different from that of U.S. Pat. No. 4,793,307 (1988).
The determination whether LIS of a certain element is technically feasible or not requires detailed consideration of a variety of issues including the photoionization pathway, the associated lasers, and the atomic vapor. A definitive evaluation of the applicability of LIS requires knowledge of the isotope shifts, hyperfine structures, level energies, angular momentum of each level, level lifetimes, etc. For those cases where LIS is technically feasible, an economic evaluation can be performed in the consideration of the cost competitiveness of this technique against other techniques, which requires knowledge of physical and chemical properties of the element, required laser power, isotope selectivity and ionization efficiency of the photoionization process, etc.
Thallium has no available autoionization levels and isotope shifts of thallium energy levels are relatively small compared with other elements. Accordingly, photoionization of thallium atoms through an autoionization level is not possible and also isotope selectivity might be very small for the conventional stepwise photoionization process, which was applied in the U.S. Pat. No. 4,793,307 (1988) and U.S. Pat. No. 5,443,702 (1995). Furthermore, it may not be possible to separate the thallium isotopes using the angular momentum selection rule applied in the U.S. Pat. No. 5,202,005 (1993) since the nuclear magnetic moments of 203Tl and 205Tl are all the same.
Japanese patent publication No. 2000-262866A adapts the isotope-selective excitation and field ionization method for thallium isotope separation, which was disclosed in U.S. Pat. No. 5,945,649 for 210Pb removal from lead isotopes. In this patent, thallium atoms in the ground state are isotope-selectively excited to a Rydberg state at the energy of about 49,000 cm−1 by two lasers of 377.6 nm and 443.7 nm wavelengths and they are field-ionized by an external electric field. During the excitation process, detuning of about 1 GHz is given between laser frequencies and the intermediate level at the energy of 26,477 cm−1. This is a 2-color 2-photon excitation process. To obtain high isotope selectivity as well as high excitation efficiency by using 2-photon excitation process, lasers should have nearly Fourier-transform-limited bandwidth and also should counter-propagate. The Stark shift caused by detuning from the intermediate level decreases the 2-photon excitation efficiency. To avoid this, laser powers and frequencies should be controlled finely according to the π-pulse condition and also laser pulses should be overlapped exactly. Since it is practically impossible to overlap the laser pulses during the entire counter-propagation in medium, drop of the excitation efficiency is inevitable. Furthermore, for the case of mass production of thallium isotopes, initially produced plasma may screen the external electric field and consequently decreases the field ionization efficiency. For these reasons, although the Japanese patent publication No. 2000-262866A may be technically feasible, there might be several obstacles to overcome for this method to be commercially practical.
Hence it is an object of the present invention to solve the above mentioned problems and also to provide an improved practical thallium isotope separation method.